There is known a tap with a prepared hole cutting tool in which a prepared hole cutting tool having drilling edges for cutting or drilling a prepared hole, and a tap portion are formed coaxially and integrally. The tap portion is provided with a male thread corresponding to a female thread to be formed, and thread cutting edges formed along chip discharging flutes provided to divide the male thread. The thread cutting edges cut a female thread on the prepared hole formed by the prepared hole cutting tool. Patent Document 1 discloses such a tap with a drill, and Patent Document 2 discloses such a tap with an end-mill.
The tap with the drill is suitably employed when a female thread is cut on a solid or pure workpiece to be cut having no prepared hole. The tap with the end-mill is suitably employed when the female thread is cut on a workpiece to be cut provided in advance with a rough hole such as a cast hole and the like which is smaller than an inner diameter of the female thread to be formed.
A tap with a drill 80 shown in FIG. 6B will be specifically described. This tap 80 comprises a tap portion 84 provided with one streak male thread 82 and a drill 86 as a prepared hole drilling tool, which are formed coaxially and integrally. The drill 86 and tap portion 84 are continuously formed with unillustrated chip discharging grooves (helical flutes) at a constant lead, and drilling edges 87 are provided at a portion where the chip discharging flutes open at the tip end of the drill 86. The tap portion 84 is provided with thread cutting edges 83 along the chip discharging flutes.
As shown in FIG. 6B, after cutting a prepared hole 90 on a workpiece 88 to be cut by a proceeding drill 86, the tap portion 84 cuts a female thread 92 on a prepared hole 90. In this case, since a diameter dimension (drill diameter) Dd of the drilling edges 87 of the drill 86 is selected larger than a valley diameter d1 of a male thread 82 of the tap portion 84, a diameter dimension Dh (≈Dd) of the prepared hole 90 formed by the drill 86 directly corresponds to an inner diameter D1 of the female thread 92. Further, a valley diameter D and an effective diameter D2 of the female thread 92 correspond to an outer diameter d and an effective diameter d2 of the male thread 82 of the tap portion 84, respectively. FIG. 6 is a set of schematic views showing an upper half of a central line O of the tap with the drill 80. It is an explanatory view for mainly explaining the diameter dimensions of respective portions of the cutting edges, with the chip discharging flutes omitted. FIG. 6A represent rotation locus profiles of the drilling edges 87 and the thread cutting edges 83 formed at the opened portion of one chip discharging flute.
Here, when the female thread 92 is cut on a through-hole, a length of the drill 86 is generally selected longer than that of the female thread 92. First, the tap with the drill 80 is rotationally driven at high rotation speed suitable for drilling the penetrated prepared hole 90 by the drill 86. Then, feeding of the tap with the drill 80 is switched over to a synchronous feeding in which the tap portion 84 screws into the prepared hole in accordance with the lead of the male thread 82 of the tap portion 84, so that the thread cutting edges 83 of the tap portion 84 cut the female thread 92. FIG. 6B shows a halfway state in which the tap portion 84 cuts the female thread 92 on the prepared hole 90.
On the other hand, when a female thread 94 is cut on a blind hole i.e., axially closed hole as shown in FIG. 6C by the short drill 86, the tap is fed in the synchronous feeding from the beginning so that the tap portion 84 screws into the prepared hole in accordance with the lead of the male thread 82 of the tap portion 84. With cutting the prepared hole 90 by the drill 86, the thread cutting edges 83 of the tap portion 84 cut the female thread 94.
[Patent Document 1] Japanese Patent Laid-Open No. 10-100020
[Patent Document 2] Japanese Patent Laid-Open No. 10-86019